or you can edit it using gedit.
taufanlubis@zyrex:~$ sudo gedit /etc/papersize

When you change the value in /etc/papersize, it will automatically change the value in the paperconf.
You have to exit your applications before you can use it.

Change I change the paper size from one of my applications?Yes, you can. For example, you use a4 for all applications except letter for wordprocessor. Just, change mannuallyinside the word processor application before you start, edit or print your new document.